In recognition of the importance of needle sharing as a causal factor in acquiring viral infections and later developing cancer, a collaborative project has been undertaken with researchers at the Johns Hopkins University ? School of Hygiene and Public Health who evaluate the Baltimore Needle Exchange Program (NEP). Needle sharing is a common means by which intravenous drug users (IVDUs) acquire viral infections. Those infections are most commonly with hepatitis B, hepatitis C and HIV-1. The hepatitis viruses are causative agents in liver cirrhosis and hepatocellular carcinoma and HIV-1, as the causative agent of AIDS, has cancer outcomes of lymphomas, Kaposis sarcoma and other rare cancers. In the initial experiments, syringe content samples were genotyped and analyzed for the short tandem repeat (STR) locus D6S502 using customized methods. Experiments were conducted blind to whether or not syringe contents represented single or multi-person use on a panel of reference samples. Following polymerase chain reaction (PCR) amplification and gel electrophoresis, electropherograms were generated to ascertain the pattern of alleles present in each individual needle wash. In FY99 a total of two test panels have been examined. In the most recent test panel (N=41), inspection of electropherograms was conducted to classify syringe specimens as representing single or multi-person use based on the presence of more than two STR alleles indicating sharing. Results were then unblinded to permit comparisons to the known blood trace contents. Using two-by-two tables, sensitivity and specificity were 71% and 73%, respectively. The extent of agreement over and above chance, as indicated by calculation of the Kappa coefficient, indicated good agreement. We are confident that the sensitivity and specificity can be improved beyond 90% through the analysis of multiple loci available through a standard PCR forensics kit and accompanying software. Recently, we switched to commercial forensic kits that are available for genotypic STR analysis that will substantially improve our ability to differentiate between trace DNA from different individuals. One of those kits, AmpFliSTR (PE BioSystems, San Jose CA), identifies genotypes from nine loci (D3S1358, VWA, FGA, D8S1179, D21S11, D18S51, D5S818, D13S317 and D7S820) and a sex differentiating locus (Amelogenin) in one multiplexed PCR assay. In forensics applications, this kit differentiates between individuals with a probability of 1 x 10-11 (i.e., a product of the frequencies in the population of the alleles observed at each locus). Loci that are included in this kit were chosen to minimize PCR artifacts that can complicate interpretation of results. These features make this kit ideal for testing our field syringe specimens. These methods were developed to test several hypotheses related to needle sharing and viral infection. One, self-reports of multi-person use of syringes underestimate true levels. Two, multi-person use of syringes has significantly declined over time since the NEP was introduced. Three, NEP syringes acquired by secondary exchange will have circulated longer and will have higher rates of multi-person syringe use, compared to NEP syringes acquired by primary exchange. Four, syringe sharing can identify high-risk seronegatives for our HIV-1 genetic epidemiology studies aimed at identifying infection resistance genes. Needle Sharing: A Risk Factor for HIV-1 Infection. - Epidemiology, viral infection, intravenous drug use, HIV-1, Hepatitis B virus, Hepatitis C virus, forensics, - Human Tissues, Fluids, Cells, etc.